There has been significant interest in using carbon-based nanomaterials as chemical sensors due to advantages such as light weight, high electrical conductivity, high electrochemical surface area, and superior sensing performance. Carbon nanotubes (CNT), including single-walled carbon nanotubes (SWNT or SWCNT), are particularly attractive due to their high electron mobility and large current carrying capacity. CNT can reduce power consumption and exhibit high temperature stability and chemical inertness, providing a stable and robust platform to detect specific analytes. Chemical sensors containing untreated CNTs utilize their intrinsic electrochemical properties, which limits the sensor selectivity and sensitivity. One approach to improving selectivity has been to functionalize CNTs either covalently or non-covalently with various materials. However, owing to their one-dimensional nanostructure, CNTs are highly sensitive to environmental factors such as humidity and temperature, which can restrict their use depending on the season, region, and weather. Thus, there is a need for more selective, specific, and stable nanoscale and microscale chemical sensor devices and methods for making and using them.
Recently, nanowires, nanotubes, and nanospheres as donors of electrical responses have been studied for the minimized nanostructures in the field of biosensors. Nanoscale biosensor devices can support in-vivo applications, and provide high sensitivity and detection at low concentrations (1). In addition, research on nanoscale biosensors has attempted to simplify detection by providing label-free, rapid, low-cost, multiplexed analysis. SWNTs are attractive materials for use in nanoelectronics (2-4). In particular, the electrical properties of SWNTs are good for use in advanced biological electronics and biosensors. Assembly of SWNTs and gold onto silicon wafers enables a high sensitivity electrical response for biosensors. Electrostatic or capillary methods are usually utilized for SWNT assembly onto silicon wafers, though their attachment to the silicon wafer is weak. Therefore, methods are required to maintain intact, assembled SWNTs are required. The use of SWNT-based chemiresistive/field-effect transistor (FET) sensors has been applied to medical sensor in-vitro systems (5-6). However, FET devices require three electrodes (working, reference, and counter electrodes), and their large size is hard to apply as an in-vivo medical detection system, although such devices can provide high sensitivity detection of target materials.
Miniaturized biosensors should detect and quantify small molecules with high sensitivity and selectivity. A variety of electrode modifications have been used for the immobilization of biomolecules onto SWNTs with covalent or non-covalent bonding methods. Covalent bonding methods using SWNT modification with chemical functional groups is associated with severe problems regarding SWNT electrical properties, because such methods can change (7-8). On the contrary, non-covalent bonding methods using π-π stacking do not enable the transfer of chemical characteristics because they only utilize physical forces to immobilize materials onto SWNTs. Enzyme immobilization is also an important process for increasing the sensitivity and stability of biosensors. However, immobilized enzymes typically have low activity due to differences in local pH or electrostatic interactions at the matrix-enzyme interface, changes in overall enzyme structure resulting from covalent linkage, or matrix-induced confinement that decreases enzyme mobility available for conformation changes during substrate catalysis (9-11). Thus, there is a need to develop improved functionalization of SWNT using enzymes.
Further, there is a need to develop simple, sensitive, and stable biosensors with small footprint for the measurement of physiological markers, such as glucose, lactate, and urea in body fluid samples.